1. Field of the Invention
The present invention relates to an apparatus for detecting the occurrence of misfire in an internal combustion engine using rotational speed variation of an engine output shaft.
2. Description of Related Art
Conventionally, as such kinds of an apparatus for detecting the occurrence of misfire in an internal combustion engine, JP-A 4-365958 (U.S. Pat. No. 5,275,037), for example, discloses an apparatus for detecting the occurrence of misfire based on variation of rotational speed (a crank angular speed) between two cylinders which successively make expansion strokes. Generally, in an internal combustion engine, when a misfire occurs during an expansion stroke in one cylinder, the rotational speed at this time, that is, the rotational angular speed of the crankshaft, which is an engine output shaft, decreases. For this reason, the occurrence of misfire in each cylinder can be detected by watching changes in rotational speed.
Particularly, in such an apparatus of the above-mentioned JP-A 1-365958 (U.S. Pat. No. 5,275,037), for a 4-stroke type multi-cylinder internal combustion engine, the apparatus calculates a first variation amount based on the variation of the rotational speeds of two cylinders which successively make expansion strokes and also calculates a second variation amount in the same way based on the variation of rotational speeds of two cylinders which are separated by 360.degree. CA (crank angle) before the cylinders used for calculating the first variation. Accordingly, an occurrence of misfire in the engine is detected on the basis of the difference between the first and second variations. Therefore, to obtain the difference of rotational speed variation for cylinders which are separated by 360.degree. CA means to watch the rotational speed variation of facing cylinders (cylinders which are separated by one rotation of the crankshaft in the expansion stroke) in an internal combustion engine having an even number of cylinders. In such a case, the rotational speed variation, which is substantially consistent in a period of the rotational variation (dispersion degree), can be used as a parameter, thereby being regarded as an object which can cut down on errors in the misfire detection.
In contrast, as a technique of detecting misfire by using the above-mentioned difference between rotational speed variations, the prior art has proposed an apparatus which calculates rotational speed variation for cylinders separated by 720.degree. CA, that is, the same cylinder, and detects misfire on the basis of the calculation result. This technique has almost completely canceled detection errors caused by the dispersion of rotational speeds between cylinders.
However, according to the above-mentioned related art, problems remain. For example, the conventional misfire detecting apparatus may not be able to detect an occurrence of misfire caused by a specific pattern. More specifically, in case of calculating the difference in rotational speed variation for facing cylinders (cylinders separated by 360.degree. CA), occurrence of misfire cannot be detected because, when both facing cylinders are misfiring, rotational variations due to misfires are counterbalanced. In addition, in case of calculating the difference of rotational speed variation for of the same cylinder (cylinders separated by 720.degree. CA), an occurrence of misfire also cannot be detected because when the same specific cylinder is continuously misfiring, rotational variation due to the misfires are counterbalanced.